1. Technical Field
The present invention relates to a pulse generating circuit, suitable for an UWB (Ultra Wide Band), an electronic device using this circuit, and a method of transmitting information using this circuit.
2. Related Art
UWB communication is a communication system for performing high-speed and high-capacity data communication using a very wide frequency band. A communication system using a wideband signal includes a method using a spread spectrum and a method of orthogonal frequency division multiplex (OFDM) in the related art. However, UWB is a communication system of a wider band using very short pulses, and is also referred to as a communication of an impulse radio (IR) system. In the IR system, modulation and demodulation are possible only by operating a time axis which does not depend on modulation in the related art, and it is considered that simplification of the circuit or reduction of power consumption can be expected (see U.S. Pat. No. 6,421,389, Pub. No.: US2003/0108133A1, and Pub. No.: US2001/0033576).
A pulse waveform used in the IR system will now be described. A pulse train having a pulse width PD and a cycle TP as shown in FIG. 16A is well known and the frequency spectrum of the pulse train is a sinc function whose envelope has a first zero point at a frequency of BW=1/PD as shown in FIG. 16B.
It is not easy to use such a pulse since the spectrum spreads from a direct current to BW, and hence a pulse whose center frequency f of the spectrum is at a high position as shown in FIG. 17B is preferred.
In other words, it has a pulse waveform as shown in FIG. 17A and is obtained by multiplying the pulse in FIG. 17A by the square wave of f0=½PW to move the frequency spectrum toward the higher side. However, this waveform contains a direct current (DC) component as shown by a chain line 1701 in FIG. 17A and, precisely speaking, does not have a spectrum as shown in FIG. 17B. Various pulse waveforms which are ideal for the UWB communication other than this waveform are devised and are different from the waveform shown here. However, the waveform shown here is in heavy usage since the method of generation it is easy.
FIG. 18A shows an example of a circuit in the related art for generating the pulse as shown in FIG. 17A (A CMOS IMPULSE RADIO ULTRA-WIDEBAND TRANSCEIVER FOR 1 Mb/s DATA COMMUNICATION AND ±2.5 cm RANGE FINDINGS T. Terada et. al, 2005 Symposium on VLSI Circuits Digest of Technical Papers, pp. 30-33).
As shown in the drawing, two inverters 1801, 1802 and an NOR circuit 1803 constitute a ring oscillator of three stages when another input C of the NOR 1803 is false (L: low level) In other words, C oscillates for the period of L and change of outputs NR1, N1, N2 of the NOR 1803 and inverters 1801, 1802 is propagated by a delay of time t respectively as shown in a time chart in FIG. 18B. For simplification, it is assumed that the rising time and the dropping time of the NOR 1803 and the inverters 1801 and 1802 are all the same. Therefore, the pulse width (Pw in FIG. 17A) generated in this circuit is 3 t. In other words, three times the delay time of an element which constitutes the circuit is the shortest possible pulse width.
However, when an attempt is made to obtain a pulse of a required high frequency band by the pulse generating circuit in the related art as described above, an element having a sufficient speed must be used. However, in fact, it is very difficult or impossible to obtain such an element.
Generally, when an attempt is made to operate the element at a high speed, an increase in power consumption results. Therefore, when an attempt is made to obtain a very short pulse in such a circuit, an increase in power consumption is inevitable in the related art. In addition, reduction of power consumption is also strongly desired in a case in which transmission of a signal is desired to be carried out wirelessly in a very short distance as in the case between a plurality of housings divided and connected so as to be capable of relative displacement in terms of posture and position, or in the same housing.